Fiber optics are widely used in many diverse applications, including telecommunication systems, instrumentation and sensing operations. An example of such an application is a multi-access optical telecommunications network. In such a network, optical fiber connects a number of users or subscribers to a central office using passive couplers. This type of network is particularly attractive since there are typically no active optical devices located outside of the central office or subscriber locations.
An optical fiber typically includes an inner core region surrounded by an outer cladding made of a similar material. The inner core has a relatively higher index of refraction than the cladding, resulting in total internal reflection of the light beam within the core. This results in very efficient transmission of light through the core. Light may be transferred or split between separate fibers through the use of a fiber optic coupler. One extensively used type of fiber optic coupler is a fused biconically tapered (FBT) coupler. In one method of producing such a fiber optic coupler, a number of optical fibers are held in axial alignment and elongated while being heated. This process creates a biconically tapered region or waist wherein the optical fibers are fused together and the optical signals from one or more optical fibers can be coupled to or split between the other optical fibers.
The basic optical performance of fiber optic couplers can be described by three fundamental quantities: excess loss, insertion loss and uniformity. The excess loss, expressed in decibels (dB), is a measure of how much light or optical energy is lost in the coupling process. Excess loss is defined as the ratio of the total output power to the amount of optical power launched into the input fiber of the coupler. The ratio of the optical power in one of the output fibers relative to the input optical power is known as the insertion loss. The insertion loss is also often expressed in decibels. Another term often used to characterize the optical performance of couplers is uniformity. Uniformity is a measure of the spread in the insertion losses of the coupler. It is also expressed in decibels and is defined as the difference between the maximum and minimum values of insertion loss.
Many of the fiber optic couplers in use today are designed to operate effectively over only a narrow range or "window" of wavelengths. The most common wavelengths of interest for telecommunication applications are those centered around 1310 nm and 1550 nm. These fiber optic couplers, often called single window couplers, essentially provide equal splitting of light from one or more input fibers to a number of output fibers at a preselected wavelength. The insertion loss of each output port for such a single window coupler changes as a function of the wavelength of the transmitted light. In particular, as the wavelength of the transmitted light varies from the center of the wavelength window, the optical power in the output fibers (i.e., insertion loss) tends to diverge from the ideal value and the uniformity becomes quite large. This behavior typically limits the use of such single window couplers to within .+-.20 nm of the center of the wavelength window.
In many optical fiber telecommunication applications, simultaneous operation within both 1310 nm and 1550 nm wavelength windows is required in order to provide both telephony and broadband services. In these applications, broadband fiber optic couplers, which exhibit a relatively constant insertion loss over a broad range of wavelengths, are required.
Traditionally, 2.times.4 couplers have been fabricated by concatenating three 1.times.2 or 2.times.2 couplers to form the desired configuration. The resulting tree structure has a fairly large package size, which can be a problem when the space allotted for the coupler is limited.
Recent advances have allowed 2.times.4 couplers to be built without the need for concatenation. For example, see U.S. Pat. No. 5,355,426 to Daniel et al., which discloses a process, wherein four fibers of mismatched diameters are fused at a single point to achieve a broadband response. This technique basically requires three different fiber diameters to be created prior to fusion. A difficulty with this technique is that precise fiber diameter ratios must be maintained for optimum performance. Variations in the diameter ratios will cause an increase in uniformity, and the diameter ratios can be difficult to control when three different fiber diameters are required.
The present invention overcomes these and other problems and provides a 2.times.4 broadband coupler and a method for producing same, which apparatus and method utilize two fiber diameters, and provides a coupler which is easier to manufacture and which has improved operating characteristics.